The present invention relates to a rotor for a superconducting rotating electric machine. More particularly, it relates to a rotor for a superconducting rotating electric machine in which the superconducting field coils are more reliably secured to the rotor.
Due to the very high speeds of rotation of a rotor for a superconducting rotating electric machine, the superconducting field coils of such a rotor are subjected to very high centrifugal forces. Since any movement of the field coils may not only result in their damage but may generate frictional heat which can cause a loss of superconductivity, it is extremely important that the coils be rigidly secured to the rotor.
FIGS. 1 through 5 illustrate a conventional rotor for a superconducting rotating electric machine of the type disclosed in Japanese Laid Open Patent Application No. 57-166839, of which FIG. 1 is an overall cross-sectional view. As can be seen from FIG. 1, the rotor has a cylindrical torque tube 1 in the middle of which is formed a coil-carrying shaft 2. The outer periphery of the rotor is defined by a cylindrical warm damper shield 4, which is rigidly secured at either end to an outboard shaft 8 and an inboard shaft 9, the inboard shaft 9 being connected to an unillustrated turbine or load, depending upon whether the rotor is used as part of a generator or a motor. Both of the shafts 8 and 9 are journaled in bearings 10. The inboard shaft 9 has slip rings 11 formed thereon by which current is supplied to superconducting field coils 3 mounted on the coil-carrying shaft 2. A cylindrical cold damper shield 5 is secured to the torque tube 1 between the coil-carrying shaft 2 and the warm damper shield 4. The damper shields 4 and 5 serve to shield the superconducting field coils 3 from alternating current magnetic field, and also serve to damp low frequency oscillations of the rotor during disturbances of the electrical system to which the rotor is connected. Liquid helium, whose flow is indicated by the arrows, is supplied to the inner cavity of the coil-carrying shaft 2 and to heat exchangers 12 formed in or mounted on the torque tube 1 by unillustrated piping. The inner cavity of the coil-carrying shaft 2 is hermetically sealed by an outer tube 6 secured to the outer periphery of the coil-carrying shaft 2 and by end plates 7 secured to the ends of the coil-carrying shaft 2 so that liquid helium introduced into the cavity will not spread to other parts of the rotor. Thermal radiation shields 13 which protect the field coils 3 from lateral radiation are mounted on the torque tube 1 at the ends of the coil-carrying shaft 2. The portions indicated by reference numeral 14 are evacuated.
As shown in FIG. 2, each of the superconducting field coils 3 comprises parallel straight portions 31 which extend parallel to the axis of the coil-carrying shaft 2 in which it is mounted, arcuate portions 32 formed at the ends of the straight portions 31 which extend circumferentially over the coil-carrying shaft 2, and corners 33 which connect the straight portions 31 and the arcuate portions 32.
As shown in FIG. 4, which is a cross-sectional view taken along Line A--A of FIG. 1, the coil-carrying shaft 2 has a number of parallel longitudinally-extending coil slots 18a machined therein in which the straight portions 31 of the field coils 3 are housed. The coil slots 18a are separated from one another by rotor teeth 2a which extend radially outward from the longitudinal axis of the coil-carrying shaft 2. Wedges 15 are inserted into wedge grooves formed in the rotor teeth 2a so as to restrain the straight portions 31 of the field coils 3 housed in the slots 18a against centrifugal forces. Each of the coils 3 is surrounded on its bottom and sides by longitudinally-extending electrical coil insulation 19 and on its top by wedge insulation 20. For the purpose of better illustrating the structure, the coil 3 and insulation for the leftmost of the slots 18a in FIG. 4 have been omitted.
As shown in FIG. 5, at each end of the coil-carrying shaft 2 is a section with a reduced outer diameter. In these sections the arcuate portions 32 and the corners 33 of the superconducting field coils 3 are housed in wide circumferentially-extending slots 18b which join the ends of the axially-extending slots 18a (not shown in FIG. 5). While each of the axially-extending slots 18a houses the straight portion 31 (not shown in FIG. 5) of only a single coil 3, the circumferentially-extending slots 18b each house the arcuate portions 32 and corners 33 of a plurality of field coils 3. The bottom surface of these slots 18b is covered with bottom electrical insulation 21 on which the coils 3 sit, and the tops of the coils 3 are covered by a cylindrical electrically-insulating cover 22. A retaining ring 16 is shrink-fit over the insulating cover 22 so as to restrain the coils 3 against centrifugal forces. Between each of the coils 3 and between the coils 3 and the sides of the slots 18b, electrically-insulating packing 17 is disposed which serves to insulate the coils 3 from one another and to prevent their sideways movement.
However, the electrically insulating packing 17 between the arcuate portions 32 of the field coils 3 has a coefficient of thermal expansion which is about twice as large as that of the coil-carrying shaft 2 or the field coils 3. Therefore, while it is possible to rigidly secure the field coils 3 in the circumferentially-extending slots 18b at normal temperatures, when the rotor is cooled to extremely low temperatures during operation, gaps develop between the arcuate portions 32 of the field coils 3 and the electrically insulating packing 17. As the electrically insulating packing 17 is not secured to the slots 18b in the coil-carrying shaft 2, it is possible for the arcuate portions 32 of the field coils 3 to move due to the gaps, producing frictional heat which may cause a loss of superconductivity.
An alternative method of securing field coils to a rotor which has been used in the past is to house not only the longitudinally extending portions of the field coils but also the arcuate portions of the coils in individual slots in the rotor. The arcuate portions of the field coils are held in the slots by wedges, just as are the longitudinally-extending portions. While such a structure can secure the coils against movement, it is impossible to install a previously-wound field coil into the slots in the rotor. Rather, the field coils must be wound inside the slots, which makes their installation extremely time-consuming and expensive.